In our June 2021 article, “Heath Care Design: FGI Guidelines, ANSI/ASHRAE/ASHE Standard 170, and Beyond,” we reviewed the codes and standards for health care environments that form the guidelines to be applied at the beginning of a health care facility design. The article highlighted that these minimum requirements are just that — the bare minimum — and indicated the application of these guidelines only started the process for creating safe and comfortable environments conducive to promoting good health care and lowering risks of infection. We challenged current and future mechanical engineers to design past bare minimums and educate clients on the real “value” of designing systems that more appropriately meet the expressed “purpose” of ASHRAE Standard 170. The previous article referenced the quote: “When a flower doesn’t bloom, you fix the environment in which it grows. Not the flower.”1 We, as engineers, need to expand our thinking beyond the status quo and seize the initiative to play a more pivotal role in creating health care facilities that support building environments conducive to good occupant health, personal wellness, and patient healing or recovery.

Facilities are often designed by architectural teams that conform to owner design requests and to a specified budget. Consequently, current mechanical designs are often “shoehorned” into building layouts and predetermined budgets; a design that only maintains desired conditions prescribed as minimal requirements by the Facility Guidelines Institute (FGI) or ASHRAE Standard 170. However, the human cost associated with bare minimum designs are frequently not considered. Hospital-acquired infections (HAI) have long been a part of our world and have been a catalyst to better designs. The pandemic has motivated reassessment of buildings, resulting in engineering designs that more effectively reduce the risk of infection. System resiliency and emergency/pandemic response preparedness have been thrust into the spotlight. However, HAIs haven’t gone away. They remain and draw even more attention to the need for creating healthier built environments. While looking at mechanical systems to address resiliency and pandemic preparedness, we must also enhance designs to eliminate the threat of HAI’s, not ‘first cost’. Unlike the financial considerations, the human cost expands far beyond the monetary factor.
        Consequently, mechanical systems should play a more fundamental role in creating healthier environments by providing fresh air, removing airborne contaminants, maintaining proper humidity levels, and promoting asepsis (the absence of bacteria, viruses, and other microorganisms). One could say, HVAC systems are silent partners in securing health care objectives. Patients, visitors, and health care workers are often unaware how important mechanical systems are in keeping them safe. Occupants expect systems to be designed properly, installed correctly, and, quite simply, to work. To allow first cost to be the driving factor does nothing more than support a concept that designing an OK health care environment is just that, OK, and begs the following questions;

  • Do you want to be treated by a doctor that is just OK at performing surgery?
  • Do you want to drive a car with tires that have just enough traction to hold you to the road?  
  • Do you want to fly in an airplane that is just OK and has the minimum number or rivets holding the wings together?”
FIGURE 1: GLHN: room geometry and occupants.

The answer to all of these questions is, of course, no. Consequently, especially now, considering the pandemic, owners and engineers now should more carefully evaluate the human factor when considering how best to design HVAC systems. To do otherwise, solely on first-cost pressures, is shortsighted, at best. Designing to code minimum also fails to take into account another relevant design consideration: the ambitious and ever-expanding efficiency objectives prescribed by ASHRAE Standard 90.1 and the IECC codes.

This article will evaluate three HVAC system concepts that offer the promise of creating superior IAQ and healthier environments while creating the opportunity for enhanced operating efficiency. Creating healthier environments while enhancing system efficiency do not necessarily have to contradict each other, providing newer approaches to heat transfer are adopted. Historically, HVAC designs apply the laws of heat transfer physics to calculate the amount of a building’s total load and to assign the amount of consumed energy required to do so. However, what if engineers considered a more expanded view of how the laws of physics might be applied. What if engineers applied systems that "work with" physical laws, not against them, and thereby assume more of the work to be done?

An Aside: Overhead 'Mixed-Air' Systems

To demonstrate this point, let’s evaluate a traditional overhead “mixed-air” system. To maintain room temperature and humidity set points, air is injected at a high velocity from supply air ceiling diffusers or high in-wall registers. High-velocity discharge from these devices causes room air to be induced into the supply air jet, creating a “theoretical” mixed-air room condition, i.e., uniformity of temperature throughout the cubic volume of space. However, what if air distribution is rethought. Instead of using greater fan energy required for this approach, what if a design applied the principles of convection, i.e., warm air rises, cold air falls, to move air within a space. Such an approach, otherwise known as displacement ventilation (DV), would allow system static pressures to be reduced. The characteristics of displacement airflow will be reviewed in depth later in this paper.

Before doing so, let’s first review current findings regarding overhead systems. Recent research challenges previous concepts on how airflow and particulate distribution in mixed-air environments actually occur. It has been demonstrated that properly designed mixed-air systems result in temperature uniformity within the cubic volume of space to within a couple degrees. In 2015, the ASHRAE Journal published an article, “The Basics of Well-Mixed Room Air Distribution.”2 Under the Return Air Inlets section, a comment is made, “At the airflow rates present in most well-mixed spaces, the location of the return air inlet has a negligible effect on air movement in the room.” Though correct in reference to room temperature profiles and thermal comfort, such a view does not necessarily represent particle motion. Outside of Brownian motion, air particles are driven by air motion. Recent computational fluid dynamics (CFD) modeling, generated by an Arizona engineering team3, illustrates that, depending on the environment and function of the space, some constant volume systems do not achieve a truly mixed-air environment with uniform contaminant dispersion. Room geometry, furniture location, and the position of occupants in the space (see Figure 1) often causes room air vortices to form creating regions where contaminants and pathogens accumulate and are not effectively drawn out of a room to be recirculated through the filtration/return/exhaust system. (see Note 1 and Figure 2).

FIGURE 2: GLHN: elevation-room air velocity profile.

Consequently, when germicides are expressed within a room, other CFD models illustrate that tactical placement of more return grilles and supply diffusers can more effectively restrict the dispersion of released pathogens and remove them. Dr. Kishor Khankari published an article in the July 2021 issue of ASHRAE Journal titled, “Analysis of Spread of Airborne Contaminants and Risk of Infection,”4 that concluded, from such CFD findings, the following compelling statements:

  • “… A single, four-way supply diffuser and a single return grille can promote the formation of stagnant air recirculation zones, which can form pockets of high concentration of contaminants.”
  • “Create a distributed supply layout by increasing the number of supply diffusers and strategically placing them over the occupied zone.”
  • “Create a distributed return layout by increasing the number of exhaust outlets to create a path of least resistance for the contaminated air to exit the space.”

Dr. Khankari’s article offers the following conclusion; “…airflow sub-zones should be added to a room to control pathogenic migration and removal.”

Conceptual mixed-air environments should also be evaluated under more common conditions, e.g., variable air volume (VAV) applications at part load and/or supplying warmer air during heating mode. VAV systems in cooling or heating mode, when zone loads are satisfied in non-critical care areas, reduce the amount of supply air. Consequent low-velocity supply air jets tend to dump air into a space, further compromising mixing and air distribution effectiveness. During the colder times of year, also known as the cold and flu season, supply diffuser air volume is decreased for VAV systems, and when in heating mode, higher diffuser supply air temperatures increase the chance of air short circuiting into return and exhaust grilles, thereby compromising thorough mixing. Owners and design teams should recognize and evaluate these phenomena more critically, especially now that ASHRAE, the U.S. Centers for Disease Control and Prevention (CDC), and the World Health Organization (WHO) have formally acknowledged that aerosolized SARS-CoV-2 is a major means of pathogenic transmission. 

Displacement Ventilation

DV is a method of air distribution that effectively applies the principles of convection (hot air rises, cold air falls) to move heat from the occupied zone to the upper levels of a room. In 2009, a study was published by HVAC&Research to compare DV and a mixed-air system in a hospital patient room.5 Supply airflow rates were based on 4 and 6 air changes per hour (ACH), and report findings resulted in the following conclusions; “…displacement ventilation with a lower ventilation rate (4 ACH) can provide an equivalent level of air quality at the breathing zone as mixing ventilation at a higher ventilation rate (6 ACH). Displacement ventilation with a high exhaust and 6 ACH provided the best air quality in the breathing zone…”

This study draws the conclusion that at 4 ACH or 6 ACH, DV creates a healthier air condition than an overhead mixed-air system at 6 ACH. As reviewed in our June article, unlike conventional mixed-air systems that inject air at high velocity into a zone at 55°F, low in-wall DV diffusers “pour” approximately 65° dry bulb (DB) air into a space at low velocity. For rooms designed at a 75° DB set point and supplied with 65° air, supply air falls to the floor, via convection, to the lower region of the room. Within a short period of time, a thermally stratified environment develops. Rooms with 9-foot ceilings, depending on room loads, will see a thermal gradient of approximately 70° at the floor, 75° at the thermostat, and 80°-85° air in the upper levels of the room where return or exhaust grilles are located.

Any heat source with a surface temperature greater than 75° will create an upward convective flow of air local to the heat source. Conditioned air effectively is drawn to the room load, i.e., the heat source in the space. A human occupant with an average body temperature of 98.6° will generate an average flow velocity of 30 fpm across the length of their body; i.e., approximately 2 fpm nearer at the feet and ankles and, as rising air gathers momentum, the heat differential becomes greater at the thorax and head. Upward flow rates at the breathing zone can achieve velocities approaching 50 fpm or more depending on the size of the occupant.

Consequently, a single pass of conditioned clean air that crosses occupant breathing zones and contaminants within ASHRAE's defined occupied zone is drawn to upper room levels, where it is exhausted. Recognizing the IAQ advantage displacement offers, ASHRAE Standard 62.1 has assigned low in-wall or in-floor displacement systems a 1.2 air distribution effectiveness in rooms with ceiling heights less than or equal to 18 feet, greater than a “perfect” mixed-air system that is assigned an air distribution effectiveness factor of 1.0. Consequently, for health care zones not assigned minimum OSA ACH per Standard 170, these zones, if not buildings, can be designed with less outdoor air. Energy savings and IAQ is enhanced when buildings operate in economizer mode for longer periods, resulting in fully ventilated environments that effectively flush out building air. It should be noted that research of displacement ventilation at the University of Hong Kong and University of Cordoba applied to health care critical zones has recently been performed. CFD modeling and laboratory testing using mannequins were used to evaluate a condition called “lock-up,” associated with DV at the patient's breathing zone in airborne infection isolation (AII) rooms. 6, 7, 8 Figure 4 shows the room and occupant configuration the Berlanga report that concluded in the report’s abstract, “If lockup phenomenon associated with displacement ventilation occurs above P (reclining patient), it has a low influence on contaminant exposure of HW (standing healthcare worker) because of the influence of the convective boundary layer of HW…”6.

Displacement System Advantages

  • single pass of conditioned, cleaner air through the breathing zone;
  • Effective removal of contaminants and germicides from the occupied zone;
  • DV diffusers are static regain devices an can lower fan energy;
  • Quieter than conventional mixed-air systems;
  • Superior thermal comfort; and
  • Utilizes the laws of physics and thermal plums to move clean air through the breathing zone.

Displacement System Disadvantages

  • Low in-wall displacement diffusers require more sheet metal than conventional overhead systems resulting in higher first costs and placement of diffusers. Additionally, duct drops need to be coordinated and accounted for in architectural layouts;
  • Heating with DV systems is likely to use a secondary solution depending on climate region; and
  • Conventional DV systems have a return air component. If pathogen mitigation is the primary motive, consider enhanced filtration, UV-C light technology, 100% OA systems, and/or UL1995 compliant ionization.
thermally stratified environments
FIGURE 3: Thermally stratified environments showing cool air at the floor with warmer air above the occupied zone.

100% Outside Air Systems — Active Chilled Beams

ASHRAE’s Epidemic Task Force (ETF), under its “Building Guides for Healthcare” topic, via the section titled, “Guidance on Recirculation and Increased Outside Air Fraction,” states, “Evaluate recirculation or increasing outside air fraction from design levels up to 100% based on specific surge plan.”

As referenced in our June article, the ETF prescribes in its position document on Infectious Aerosols, “Increase outdoor air ventilation and open outdoor air dampers to 100% as indoor and outdoor conditions permit.”  

The U.S. Environmental Protection Agency’s (EPA’s) website, under, Ventilation and Coronavirus (COVID-19), states, “An important approach to lowering the concentration of indoor air pollutants of contaminants, including any viruses that may be in the air, is to increase ventilation - the amount of outdoor air coming indoors.”

Since ASHRAE, the EPA, and the CDC are recommending increasing outside air fractions where possible, why would 100% outside supply air (OSA) systems not be given every consideration for use in health care facilities? Active chilled beams offer a cost-effective 100% OSA design strategy. Taking advantage of the heat transfer efficiency inherent in denser fluid mediums, i.e., water in lieu of air, and splitting the total load into its sensible and latent components local to each zone, thereby driving as much sensible load to the waterside, fan motor horsepower can be significantly reduced. Space latent loads are met by providing supply air with appropriately designed dew points. Consequently, 100% OSA systems applying active chilled beam technology can often be designed to satisfy space loads using the minimum OSA requirements assigned by ASHRAE Standard 170. Health care benefits include, but are not limited to, less horsepower to meet building loads, a significant reduction in reheat, and less building square footage than conventional mechanical systems require. Health care systems using active chilled beams can see 30%-40% energy savings when properly designed. And since these systems supply only outside air, the air from the building is continually exhausted and not returned to an air-handling unit (AHU). Many studies have shown that active desiccated pathogens have lifespans of up to 72 hours, possibly more, and can easily be reintroduced into facilities via the return air path. 100% OSA systems all but eliminate this concern. Active beams, however, create a mixed-air environment within building zones by injecting, at high velocity, a mixture of OSA and room air induced across a beam’s cooling coil. Consequently, active chilled beams, similar to conventional air devices, should be layed out using additional exhaust grilles, as mentioned in Khankari’s conclusions4 to minimize local infection. A compelling alternative to applying active beams in health care is displacement ventilation chilled beams. Displacement active beams are a low in-wall, low-velocity supply air strategy resulting in thermal stratification and, consequently, superior IAQ compared to mixed-air conditions.

100% Outside Air Systems — Active Chilled Beams Advantages

  • Health care facilities served by 100% outside air. No return air;
  • Significant energy efficiency through reduced system horsepower;
  • Superior IAQ for healthier environments;
  • Quieter systems compared to medium-pressure VAV systems;
  • A significant reduction of ductwork and air-handler dimensions for reduced mechanical footprints; and
  • Displacement active chilled beams offer another layer of superior IAQ and energy savings.

100% Outside Air Systems — Active Chilled Beams Disadvantages

  • Limited contractor experience installing these systems, resulting in inflated budgetary costs;
  • Cannot be applied to all spaces within a hospital setting due to code restrictions resulting in either hybrid system strategies or multiple system approaches.
test rooms
FIGURE 4: A plan and profile view of test rooms. Image courtesy of Elsevier.

Carson Solutions
FIGURE 5: Carson Solutions’ Model QLCI.

100% Outside Air — Passive Design

Passive decoupled-hydronic technology should be also be evaluated and given every consideration for health care facilities. These systems offer another “modality” to create healthier environments and provide greater energy savings compared to active chilled beam systems. Laying out hydronic radiant panels and/or convective sails for sensible “cooling” and/or heating or passive chilled beams (cooling coils mounted in the upper levels of a room), sensible loads are addressed locally to the zone. A thermally stratified environment is necessary to maximize heat transfer at the radiant or passive device. Consequently, displacement ventilation is the air delivery method of choice and decoupled from the sensible cooling component. As with DV, a thermally stratified environment more effectively removes contaminants and pathogens at the breathing zone (see Note 2). However, current studies are not conclusive for critical zones with large air change rates. The location of radiant and passive devices in relation to DV diffusers and consequent air patterns need to be studied. These designs, however, are conceptually engaging and offer the prospect of additional energy savings compared to active systems. As mentioned earlier in this article, DV diffusers, unlike active chilled beams, supply warmer air to rooms at 63°-68°. Providing outdoor humidity levels are within an acceptable range, air-side economizer modes can be applied for longer periods than a conventional DOAS unit supplying 55°-57° supply air. The radiant component also offers a thermal comfort advantage. Since the free area surface of radiant panels and/or sails absorb radiant energy from heat-generating surfaces within a space, e.g., perimeter walls, lighting, PCs, and human occupants with a direct line of sight to the device, radiant comfort complaints are significantly alleviated. Passive devices (ceiling panels, sails, or beams) in warmer Southwestern regions will likely be located at perimeter walls to locally address the conductive heat or chill effect. DV diffusers would likely be located low in a wall opposite the occupant’s bed. The location of exhaust grilles is being evaluated for effective exhaust of germicidal agents from the patient zone.

100% Outside Air Systems — Passive Chilled Beams, Radiant Panels or Convective Sails Advantages

  • Health care facilities served by 100% outside air. No return air;
  • Significant energy efficiency through reduced system horsepower;
  • Enhanced IAQ using DV for space ventilation;
  • Quieter systems compared to medium-pressure VAV systems;
  • A significant reduction of ductwork and air-handler dimensions for reduced mechanical footprints; and
  • The prospect of improved thermal comfort by addressing radiant space loads.

100% Outside Air Systems — Passive Chilled Beas, Radiant Panels, or Connective Sails Disadvantages

  • Limited contractor experience installing these systems resulting in inflated budgetary costs; and
  • Additional infrastructure needed to accommodate outdoor air requirements for spaces with passive systems.
Radiant Panel
FIGURE 6: A radiant panel is shown in a common area at American University, Washington College of Law - Tenley Campus: The engineer/architect on the project was SmithGroup Inc. Image courtesy of SmithGroup

Radiant Panel
FIGURE 7: TWA panels in Prince George, British Columbia.


How best to improve IAQ within health care facilities to reduce the risk HAIs and pathogenic infections by providing superior IAQ needs to be more thoroughly examined. Technologies are available today and research has been performed to support the evaluation and application of more innovative designs for both critical and noncritical zones of health care facilities. These solutions can be equal to or less than the cost of more conventional VAV/CAV systems providing the whole building design and its life cycle costs are evaluated appropriately. HVAC designs should no longer be thought of in a ‘one system type fits all’ manner.
Engineers should think beyond conventional heat transfer physics for calculating loads and the energy required to meet them. Physical laws can also be exploited to contribute to workload reduction to meet health care HVAC needs while creating environments that contribute more effectively to the health, well-being, and safety for administrative and health care professionals and the patients they medically treat. Let us again propose; “When a flower doesn’t bloom, you fix the environment in which it grows. Not the flower.”1


Note 1.) CFD analysis performed analyzed airflow paths for a typical HVAC layout with ceiling supply and ceiling returns. Results may vary as air device locations, room layout, and flow rates are adjusted.

  1. Heijer, Alexander Den;  Inspirational Speaker
  2. Int-Hout, Dan, 2015; “The Basics of Well-Mixed Room Air Distribution” ASHRAE Journal 2021
  3. GLHN Architects and Engineers, Inc.
  4. Khankari, Kishor, PH.D 2021; “Analysis of Spread of Airborne Contaminants and Risk of Infection” ASHRAE Journal July 2021
  5. Yi, Y., Xu, W., Gupta, J.K., Guity, A., Marion, P., Manning, A., Gulick, B., Zhang, X., and Chen, Q. 2009. “Experimental study on displacement and mixing ventilation systems for a patient ward,” HVAC&R Research, 15(6), 1175-1191.
  6. Berlanga, F.A.,Ruiz de Adana, M., Olmedo, I., Villafruela, J.M., San Jose, J.F., Castro, F. 2018. “Experimental evaluation of thermal comfort, ventilation performance indices and exposure to airborne contaminant in an airborne infection isolation room equipped with a displacement air distribution system.” Elsevier (Energy & Buidlings) 158 (2018)
  7. Villafruela, J.M., Olmado, I., Berlanga, F.A., Ruiz de Adana, M., (2019). “Assessment of displacement ventilation systems in airborne infection risk in hospital rooms” PLOS ONE (Research Article)  
  8. Khankari, Kishor, (2015) Active Chilled Beams for Patient Rooms (Engineered Systems Magazine)